In order to interrogate (i.e. read out) variable-resistance sensor elements, such as pressure-sensitive resistances or thermistors, an electrical test voltage is applied to the sensor element and the current resulting from the voltage applied is measured. This allows the instantaneous electrical resistance of the sensor element to be calculated, from which the variable to be measured (pressure, temperature etc.) can be determined.
A sensor device with pressure-sensitive sensors may, for example, be used in a seat occupancy detection function to control an active passenger restraint system in a vehicle. A sensor mat of this kind comprises several individual pressure-sensitive sensors integrated into the seat and distributed over the surface of the passenger seat. These sensors are connected to an analysing unit, which checks the triggered condition of the individual sensors. If the seat is occupied by a person, several of the sensors are triggered by the force exerted by the weight of a person on the seat, and this condition is recognised by the connected analysing circuit as indicating that the seat is occupied. This information is then passed on to the airbag control system.
In order to allow the sensors to be interrogated selectively, each of the sensors must in principle be connected to the analysing circuit. In order to reduce the number of connecting conductors, it is advisable to operate the individual sensors in a matrix array. This means that, for a quantity of n*m sensor elements, n row conductors and m column conductors are basically provided, each of the sensor elements being connected between one row conductor and one column conductor.
It should be noted that a matrix array of this kind constitutes a circuitry configuration. In other words, a matrix array in a real configuration does not require that the sensor elements be arranged in a regular grid layout, nor does it require that the individual connecting conductors run in straight lines parallel or perpendicular to one another.
The procedure for analysing a configuration in which sensors are interconnected in a matrix array is as follows. First the whole matrix array, with the exception of a first column conductor, is set to the same potential, e.g. to ground. A test voltage is then applied to the first column conductor, after which the current flowing through the individual row conductors is measured selectively. This allows resistance values to be determined selectively for those sensor elements which are connected between the first column conductor and the various row conductors. If this procedure is repeated for each of the column conductors, all the sensor elements can be measured selectively, one after the other. It should be noted here that, as an alternative means of interrogating the individual sensor elements, it is possible to apply the test voltage to each row conductor and measure the current passing through the sensor elements at the column conductor.
If the number of sensors in a sensor mat of this kind is to be increased, generally speaking the number of connecting conductors must also be increased. This means, for example, that if the (n×m) matrix is extended to an ((n+1)×m) matrix, another row conductor will have to be introduced to incorporate the additional sensors into the matrix array.
In practice, however, such an increase in the number of connecting conductors leads to several problems. For instance, having a large number of conductor paths leads to difficulties in designing the shape of the sensor mat. The individual sensors in a seat occupancy sensor, for example, are arranged in a sandwich structure consisting of two carrier films and a spacer which, on the one hand must form a coherent whole, but on the other hand must cover as small an area as possible if seat comfort is not to be impaired. The individual sensors are therefore connected to one another simply by narrow bridges in the sandwich structure through which the connecting conductors of the sensors must run. Increasing the number of required connecting conductors makes it more difficult to run the conductors through the narrow connecting bridges, or makes it necessary to widen the connecting bridges, which makes a sensor mat of this kind in a vehicle seat more noticeable.
On the other hand, the number of connecting conductors can only be increased if the analysis circuit has a corresponding number of inputs and outputs respectively. Indeed, each row conductor and column conductor must be connected to the analysis circuit so that either a test voltage can be applied to the conductor concerned or the current flowing through the conductor can be measured. Increasing the number of connecting conductors therefore results in a more complicated and hence more expensive analysis circuit.